To get high quality reception, communication systems, which include radio and television, require a strong signal that is not corrupted by noise or interference. One form of interference that can severely degrade reception is multipath. Multipath occurs when the transmitted signal arrives at the receiver simultaneously from more than one direction. The multiple paths are generally due to reflections of the transmitted signal from hills, buildings, etc.; they can also be the result of atmospheric phenomena. The indirect paths are longer than the direct path, and consequently, the indirect path signals arrive at the receiver later in time than the corresponding direct path signal. This makes them arrive at the receiver with a different phase than the direct path signal, and, consequently, causes distortion in both the phase and the amplitude of the received signal. This can result in deep signal strength fades, overlapping communication data, clicking, etc. Examples of multipath distortion are ghosts on TV, degraded fidelity in commercial FM stereo, and loss of data in communication links.
Designing the antenna pattern gain characteristics to reject the indirect paths by placing a null in their direction of arrival is one of the better approaches to reducing multipath distortion. This eliminates the indirect paths altogether. It is easy to accomplish when conditions are known and do not change. But in most communication situations, conditions do change. The adaptive array has been used to automatically change the antenna pattern as the conditions change.
In applying an adaptive array to the general communications problem where the direction of arrival (DOA) and the time of arrival (TOA) of the signal of interest are unknown, the comparison of the adaptive array output signal to a reference signal is well suited, of which, the least means squared error algorithm (LMS) is a simple, well known example. For optimal results, the LMS adaptive array requires a reference signal which is a replica of the signal of interest. In practice, a reference signal that is more correlated with the signal of interest than the interference signal is used.
Generation of the reference signal can pose a problem. In practice, a replica of the transmitted signal is not available at the receiver. The reference signal must be derived from the adaptive array output signal. Robert Riegler and Ralph Compton (Proceedings of the IEEE, Vol. 61, No. 6, June 1973, p. 748) have discussed the application of the adaptive array to amplitude modulated communications signals, where the adaptive array output signal is processed to generate a representation of the carrier of the transmitted signal for use as the reference signal. This approach does not require knowledge of the desired signal DOA or TOA. But this approach addresses the jammer type interference problem with which the adaptive array generally is associated, not the multipath problem.
R. T. Compton and D. M. DiCarlo (IEEE Transactions on Aerospace and Electronic Systems, VOL.AES-14, NO. 4, July 1978, p.599) and Y. Bar-Ness (IEEE Transactions on Aerospace and Electronic Systems, Vol.AES-18, No. 1, January 1982, p.115) analyze another adaptive array which uses the array output to generate the reference signal. But their system was designed to address a signal environment in which the signal of interest is received along with a wideband interference signal. They do not address the multipath problem.
Ralph Compton (Proceedings of the IEEE, Vol. 66, No. 3, March 1978, p.289) discusses an adaptive array for communication signals using a spread spectrum technique. The adaptive array uses knowledge of the spreading code to generate a reference signal. August McGuffin (U.S. Pat. No. 4,217,586) has extended this approach by utilizing the multipath in the reference signal generation. The pseudo random (PN) code based reference signal generator can keep lock even in severe multipath fading. But both these approaches require a known PN code be present in the transmitted signal to generate a reference signal.
G. H. Persinger (1977 International Conference on Communications, IEEE, Pt. III, Chicago, Ill., 12-15 June, 1977, Pp. 259-262) has used a low level PN code placed in quadrature (90 degrees out of phase) with transmitted AM signal. It is used to generate the reference signal at the receiver. The reference generation is dependent on the injection of this special signal with a known code.
Peder Hansen (IEEE Transactions on Antennas and Propagation, Vol.AP-29, No. 6 November 1981, p. 836) has placed a special modulated pilot signal in the transmitted signal to be used to generate the reference signal. This technique was used specifically to discriminate against multipath. But it does not work without the special tone.
Gayle Martin (U.S. Pat. No. 4,255,791) uses noise decorrelation to generate a reference signal for an adaptive array. This method addresses an environment where there is a large interfering signal, not the multipath environment.
In a related technology, transversal filters (single input adaptive filters) which reduce TV ghosts by signal processing (not by using the antenna pattern) use portions of the transmitted TV signal structure to generate the reference signal (Shri Goyal, others, IEEE Transactions on Consumer Electronics, Vol.CE-26, February 1980). Transversal filters remove the ghosts after the received signal has been demodulated. But, they are generally microprocessor or computer based and, consequently, quite complicated and expensive.
To summarize, the prior art is limited. It either does not address the multipath problem or its approach to a solution of the problem is complex or requires special tones or codes in the transmitted signal. And consequently, there is no effective and inexpensive method of removing multipath interference at the communications receiver.